This invention relates to a method for enhancing the corrosion resistance of aluminum and aluminum alloys by deposition of a cerium-based coating thereon. The invention has particular application for aerospace structural components such as aircraft skin, wing skin and other sheet components manufactured from aluminum or aluminum alloys, especially sheet and bulk structural pieces, or in other applications where long-term corrosion resistance is desired.
Many aerospace components are constructed from aluminum or aluminum alloys due to their superior strength to weight ratio. Aluminum and aluminum alloys, however, are subject to corrosion upon exposure to water condensed from humid air and contaminated from other sources with salt, rain, snow, ocean salt, salt applied to runways, and other environmental conditions, which can lead to catastrophic failure. Aluminum corrosion is an electrochemical process involving dissolution of metal at anodic sites according to the reaction Al→Al3+.
+3e−. At cathodic sites the reduction of oxygen and evolution of hydrogen occur according to the reactions O2+2H2O+4e−→4OH− and 2H++2e−→H2. Corrosion inhibition is accomplished by reducing the rates at which these reactions occur.
Heretofore the corrosion resistance of aluminum and aluminum alloys has been enhanced by the use of chromate, conversion coatings. A conversion coating is a coating consisting of metallic salts, such as chromate, which form during and after dissolution of a metallic element, such as chromium or aluminum, or are precipitated from salts onto a substrate. A disadvantage of chromate coatings, however, is their toxicity, as ingestion or inhalation of chromates has been determined to cause kidney failure, liver damage, blood disorders, lung cancer and eventually death. Chromium is among the Environmental Protection Agency's leading toxic substances since in its hexavalent form it is a known carcinogen and is environmentally hazardous as a waste product. Many of the major environmental laws which are in force today unfavorably impact the use of chromate materials and processes. OSHA (Occupational Safety & Health Administration) requirements permit only 1 μg/m3 of insoluble chromate in the air space per 10 hour day. The chromating processes generate large volumes of hazardous wastes. Due to the health risks and inevitable government legislation associated with the application of chromate materials and their disposal, there has been a worldwide research effort to develop alternative coatings which are technically equivalent but do not pose an environmental risk.
Corrosion resistance has also been enhanced by anodizing. However, anodizing is known to cause fatigue problems leading to failure of aluminum components.
The effectiveness of cerium salts (along with other rare-earth salts) as a potential replacement to chromates for aluminum alloys was first demonstrated in 1984 by Hinton et al. at the Aeronautical Research Laboratory of Australia. Hinton et al. found that after immersing an aluminum alloy in a solution containing cerium chloride for several days, a yellowish film was formed which provided significant corrosion protection for the alloy upon subsequent exposure to NaCl solution. Over the decade, cerium salts have attracted attention as an effective corrosion inhibitor because they are not toxic and are relatively inexpensive.
The degree of protection provided to the aluminum strongly depended on the time of immersion in the CeCl3 solution. To achieve significant protection, an immersion time of at least 100 hours was generally required, which makes this process commercially unattractive. Further studies by Hinton et al. have shown that the cerium-containing films could be produced cathodically by polarizing an aluminum alloy specimen in 1000 ppm CeCl3 aqueous solution for 30 minutes. However, this cathodic coating was inhomogeneous, had poor adhesion and provided much less protection than the film formed by immersion. Hinton attributed these problems to the presence of small holes formed in the coating by evolving hydrogen, which was overcome by electrodeposition from an organic butylcellosolve solution containing 10,000 ppm Ce(NO3)3. This cathodic film with a network of cracks exhibited a five-fold improvement in corrosion resistance over that of the uncoated alloy, but was inferior to those coatings formed by the immersion process.
The possibility of obtaining a suitable cerium dip coating more quickly by utilizing an oxidizing agent has been explored. Wilson and Hinton developed a patented process to produce Ce(IV) coatings using hydrogen peroxide. This technique involved a simple addition of 1˜5% H2O2 into a solution of 10,000 ppm CeCl3 at 50 C. A yellowish coating was readily formed on aluminum alloys between 2 and 10 minutes. The main advantage of this process was that it did not require a cathodic potential to form a coating in a reasonable time. The coating exhibits good adhesion to the substrate and to paint films. Regarding its corrosion protection, however, this coating did not perform as well as the films made by the long-term immersion process. Scanning electron microscope characterizations revealed the existence of heavily cracked regions which are considerably greater than the average thickness of the film.
Another dip process involving cerium compounds was developed by Mansfeld et al. Aluminum alloy coupons were first boiled in 10 mM Ce(NO3)3 for 2 hours, then boiled in 5 mM CeCl3 for another 2 hours. In the last step, an electrochemical treatment was applied by which the samples were polarized in deaerated 0.1 M Na2MoO4 at a potential of +500 mV vs. SCE for 2 hours. This process was successfully applied to the corrosion protection of aluminum alloy 6013-T6, which showed no signs of localized corrosion after 60 days' exposure to 0.5 M NaCl solution.
When this process was applied to aluminum alloys with higher alloy contents such as 7075-T6 and 2024-T3, less satisfactory results were obtained. Al 2024 alloys showed pitting after 1 day of exposure to the NaCl solution. Mansfeld et al. reported an improved process based on a pretreatment step. Prior to the cerium dip process, aluminum alloy 2024 or 7075 was polarized at −55 mV (vs. SCE) in a solution containing 0.5 M NaNO3 acidified to a pH of 1 using HCl, or dip in an acidic chromate solution following a 20 vol % HNO3 solution immersion for 1 minute. The modified process was reported to improve the pitting resistance of both 2024 and 7075 aluminum alloys.